Dimethylnitrosamine (DMN), a potent procarcinogen, is not mutagenic per se in Neurospora crassa conidia because the conidia seems to be deficient in the activation process for promutagens. With metabolic activation, however, this agent is an effective mutagen in this organism. With the ad-3 system of N. crassa, systematic studies are carried out on four metabolic activation systems (growth-mediated, mycelium extract-mediated, organ homogenate-mediated, and host-mediated), for converting DMN to a mutagen. In addition, DMN activation by both sexes, and by organs (liver, lung and kidney are also compared, both in vitro and in vivo.